The specific aim of the Hemostatic System Activation Study (HAS) is to investigate the level of activity of the hemostatic system in patients in the Warfarin Aspirin Recurrent Stroke Study (WARSS). The radioimmunoassay measurement of the peptide fragment F1+2, a by-product of the conversion of prothrombin to thrombin, will be the indicator of the level of activity of the hemostatic system. Whether or not antithrombotic therapy is more beneficial than antiplatelet therapy in preventing recurrent stroke, HAS will provide an analysis of the actual level of in vivo thrombin generation associated with the INR achieved in the WARSS population. A higher mean F1+2 is expected in the aspirin treated cryptogenic stroke patients as compared to the aspirin treated non-cryptogenic stroke patients. A lower mean F 1+2 in the warfarin treated patients as compared to those on aspirin is expected. Some patients, however, may be resistant to the antithrombotic effect of warfarin as measured by a high F 1+2 value when their INR is in the WARSS therapeutic range of 1.4 to 2.8. The INR should correlate inversely with the F 1+2 and HAS will assess whether that relationship is constant over time. The association between F 1+2 and vascular risk factors (age, sex, hypercholesterolemia, hypertension, diabetes, smoking) and primary and secondary stroke subtypes will be studied. Other aims are to correlate F 1+2 level with data obtained in the APASS, PICSS and GENESIS substudies of WARSS, thereby enhancing the information obtained from them.